GB2130391A

GB2130391A - Ophthalmic lens having a progessively variable focal power determined taking convergence of eyes into account

Info

Publication number: GB2130391A
Application number: GB08305240A
Authority: GB
Inventors: Akira Kitani
Original assignee: Hoya Lens Corp
Current assignee: Hoya Lens Corp
Priority date: 1982-11-12
Filing date: 1983-02-25
Publication date: 1984-05-31
Also published as: NL8300742A; FR2536180A1; IT8367228A0; BE896041A; JPS6247284B2; NL184389C; SE8301087D0; DE3307009A1; IT1158813B; NL184389B; GB2130391B; SE8301087L; JPS5988718A; GB8305240D0; FR2536180B1

Abstract

An ophthalmic lens (Q) has a progressively variable focal power, taken into account of the convergence of the eyes, wherein amount of a horizontal displacement of an umbilical meridian curve (M-M') has a relationship proportional to an additional refracting power (D) at respective positions of said horizontal displacement. <IMAGE>

Description

SPECIFICATION Ophthalmic lens having a progressively variable focal power determined taking convergence of eyes into account This invention relates to improvements in ophthalmic lenses for presbyopia having progressively varying focal power, and more particularly to an ophthalmic lens of the kind above described in which the convergence of the eyes is taken into account.

Presbyopia designates such state of the eyes of a man that the eye lens in the eyehall is no more capable of adjusting itself to focusing necessary for near vision due to the loss of its original elasticity. Therefore, he will be able to easily see an object located at a short distance from him again when he wears a convex lens which makes up the shortage of accommodation.

It is customary that near vision is done through the lower areas of lenses mounted in a spectacle frame.

Therefore, a single pair of spectacles can make necessary visual power compensations for both the near vision and the distance vision when the lower areas of the conventional lenses for distant vision in the spectacle frame are replaced by the convex lenses described above.

A bifocal lens is a simplest form of such a multifocal ophthalmic lens. The convex lens portion for near vision in the multifocal ophthalmic lens is called the segment, and there are a variety of kinds in the shape, location, material, etc. of the segment.

However, the lenses of this kind have had such a common defect that, during transition of vision from distant vision to near vision, there occurs an abrupt change in the image magnification resulting in a sense of physical confusion. A so-called progressive power ophthalmic lens, which is an ophthalmic lens having a progressively varying focal power, has been proposed in an effort to alleviate the abrupt change in the magnification of the image. According to the so-called progressive power ophthalmic lens, the lens surface design is such that the refractive power is progressively varied to eliminate the sense of physical confusion experienced during transition of vision between the distant vision and the near vision, and the field of intermediate vision can also be provided in the region of the boundary between the distant vision and the near vision.

This progressive power ophthalmic lens is also aesthetically advantageous over the conventional bifocal lens in that the boundary line separating the lens portion for near vision from that for distant vision is not conspicuously sensed on the external appearance compared with the bifocal lens, and, thus, it is not perceived as that specifically prepared for presbyopia.

The progressive power ophthalmic lens is featured by the presence of a succession of "umbilical points" forming a so-called "umbilical meridian curve" extending substantially from an upper central portion to a lower central portion of the lens surface. This "umbilical meridian curve" is such that astigmatism therealong is almost equal to zero, and the refractive power varies progressively according to a predetermined rule. The term "umbilical point" is used herein to designate the point at which two major radii of curvature are equal to each other.

This major radius of curvature is a mathematical term used for expression of the property of a curved surface and has the meaning which will be described presently.

In Figure 1, the symbol S designates a curved surface, and P designates a point on this curved surface S.

The symbol e designates a normal line at the point P on the curved surface S, that is, a straight line passing through the point P and penetrating orthogonally the curved surface S at the point P.

When a plane including this normal line t intersects the curved surface S, a curve called herein a sectional curve is defined therebetween, and there are an infinite number of such sectional curves passing through the point P. Among the radii of curvature of each of those sectional curves passing through the point P, the maximum one and minimum one are referred to as the two major radii of curvature at the point P. When these two major radii of curvature are equal to each other, the point P is called the "umbilical point".

Therefore, a spherical surface is only one of curved surfaces where any one of points on the surface provides the "umbilical point", and, in this case, any one of curves on the curved surface provides the "umbilical meridian curve" described already.

When the point P is the "umbilical point", the surface portion in the vicinity of this point P is considered to be substantially spherical, and it can be said that the astigmatism at this point P is equal to zero. The term "astigmatism" is used herein to designate the difference between the aforesaid two major radii of curvature when they are replaced by the refractive powers. The radius of curvature can be converted into the refractive power (the unit of which is diopters) by the following equation well known in this field of art: R R where D is the refractive power whose unit is diopters, R is the radius of curvature whose unit is meters, and N is the index of refraction of the lens, which has no unit.

The above explanation has described that the "umbilical meridian curve" extends substantially from an upper central portion to a lower central portion of the surface of the progressive power ophthalmic lens.

The astigmatism along this "umbilical meridian curve" is almost equal to zero as described already, and the "umbilical meridian curve" possesses the most preferred optical property on the surface of the progressive power ophthalmic lens. Thus, it is quite natural that the "umbilical meridian curve" should be disposed at the position of highest frequency of use.

It is a primary object of the present invention to provide a novel and improved ophthalmic lens having a progressively variable focal power in which amount of the horizontal displacement of the "umbilical meridian curve" has a relationship proportional to the additional refracting power at respective positions of the horizontal displacement.

The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments thereof taken in conjunction with the accompanying drawings, in which: Figure 1 is a diagrammatic view illustrating the "umbilical point" and major radii of curvature at that point; Figure 2 is a diagrammatic view showing the relative positions of a visual target, eyeballs and an ophthalmic lens for illustrating the desired arrangement of the "umbilical meridian curve" on the ophthalmic lens surface and the distribution of an additional refractive power; and Figures 3(A) and 3(B) are an elevation view of an embodiment of the ophthalmic lens according to the present invention and a diagrammatic view showing the arrangement of the "umbilical meridian curve" and the distribution of the additional refractive power according to the present invention respectively.

For the purpose of finding the desired arrangement of the "umbilical meridian curve" on the surface of an ophthalmic lens, consider now the case in which this "umbilical meridian curve" coincides with the locus of movement of the fixation lines on the lens surface when the spectacle wearer turns gradually his eyes for near vision to see a visual target disposed near at a slightly lower position in front of him from the condition seeing an object disposed at an infinitely distant position in front of him. The convergence of the eyes defines the function of concentrating the fixation lines of the eyes toward a visual target in the case of binocular vision.

In Figure 2, the symbols OR and 0L designate the centers of rotation of the eyeballs of the right and left eyes respectively, the symbols CR and CL designate the apexes of the corners of the right and left eyes respectively, and the symbol I designates an infinitely distant point in front of the wearer. (Because of the limitation of the space, the direction of such a point I is shown by the arrow only).

The symbol T designates the position of a visual target disposed at a finite distance in front of the wearer in the case of binocular vision, P designates the point of intersection between the line TOR and the lens surface, F designates the foot of a perpendicular drawn from the point P toward the line IOR, and G designates the foot of a perpendicular drawn from the point Ttoward the line 0L0R Since the lines IOR and TG are parallel with each other, the angle < GTOR is equal to the angle < PORFto each other. These angles are defined as 6.

When now the right eye is turned from the state seeing the visual target T from the state seeing the point the fixation line of the right eye moves by a distance PF on the lens surface due to the convergence of the eyes.

The adjusting power of the eyeball, the refractive power of the lens for distant vision and the additional refractive power of the lens at the point P on the lens surface are defined as De, Df and Dp respectively. Then, since the distance between the eyeball of the right eye and the visual target T is TCR, Dp must be Dp=Df+ + -De .. (1) TCR (The unit of the refractive power is the diopters, and the unit of the distance is meters. The same applies to all of expressions which follow).

Further, sin 6= ORG ~~~~~ pOR = Th' and the relation TCR > CROR holds generally. Therefore, the expression providing sin 6 can be approximated to be ORG ~ PF TCR POR Therefore, the power increment D at the point P is expressed as D = Dp - Df = TCR - De PF De -- (2) ORG X POR - De ... (2) From the expression (2), PF is given by PF = ORG x POR x D + ORG X POR X De ... (3) In the equation (3), ORG, POR and De can be considered to be substantially constant. Let PF be PF = H, A a proportional constant and B a constant.Then, the equation (3) can be expressed as H =.A x D + B ... (4) Thus, in order to obtain the optimum arrangement of the aforesaid "umbilical meridian curve" and the optimum distribution of the additional refractive power, it is most desirable to satisfy the relation H=.Ax D+ B where D is the additional forcal power along the "umbilical meridian curve", H is the amount of the displacement toward the nasal side relative to the lens portion provided for distant vision, A is the proportional constant, and B is the constant.

Although the above description has referred to the right eye only, it is apparent that the same applies also to the left eye.

Figure 3 and Table 1 show an embodiment of the opthalmic lens according to the present invention.

Referring to Figure 3(A), the symbol Q designates a lens for the right eye when viewed from the convex surface side, 0 designates the geometrical center of the lens Q, L-L' designates a meridian curve passing through the point 0, M-M' designates an umbilical meridian curve passing through the point 0, and N designates a point providing a maximum additional refractive power on the umbilical meridian curve M-M'.

An H-coordinate axis (representing the amount of the horizontal displacement which corresponding to the distance PF in Figure 2) extends horizontally rightward from the origin 0, and a V-coordinate axis (representing the vertical displacement) extends vertically downward from the origin 0. The H-coordinate and V-coordinate of the point N have values Hmax and Vmax respectively.

A graph of Figure 3(B) shows how the additional refractive power D varies along the umbilical meridian curve M-M'. A D-coordinate axis (representing the additional refractive power) extends horizontally rightward from an origin 0' corresponding to the origin 0 in Figure 3(A), and a V-coordinate axis (representing the vertical displacement) extends vertically downward from the origin 0'. The D-coordinate of a point N' corresponding to the point N in Figure 3(A) has a value DMaX. This Dmax is commonly called the addition and is set at 1.00 diopter in the present embodiment.

Table 1 shown below tabulates the values of the D-coordinate and H-coordinate on the umbilical meridian curve M-M', corresponding to the values of the V-coordinate measured at intervals of 2 mm, when the values of Vmax and Hmax are Vmax = 12.0 mm and Hmax = 2.5 mm respectively.

TABLE 1 V-coordinate 2 4 6 8 10 12 D-coordinate 0.07 0.25 0.50 0.75 0.93 1.00 H-coordinate 0.2 0.6 1.3 1.9 2.3 2.5 It will be apparent from Table 1 that the values of A and B in the aforesaid expression H A x D + B are selected to be A = 2.5 and B = 0.0 in the embodiment of the present invention.

The lens for the left eye is symmetrical with or a mirror image of the lens for the right eye. That is, their refractive surface configurations are the same in the vertical direction but symmetrical with each other in the horizontal direction.

In the aforesaid embodiment of the present invention, the invention is applied to the entirety of the umbilical meridian curve M-M' on which the distribution of the additional refractive power ranges from the value of 0.0 to the addition which is 1.0. However, the present invention may be applied to such a portion of the umbilical meridian curve M-M' on which the additional refractive power varies up to at least 80% or more of the addition. Therefore, an ophthalmic lens, in which the present invention is applied to the portion of the umbilical meridian curve M-M' on which the additional refractive power varies up to at least 80% or more of the limit of power addition, is also included in the scope of claims of the present invention.

The terms "umbilical point" and "umbilical meridian curve" used in the specification are merely mathematical definitions in the strict sense, and it is true that such a point and such a curve can not be completely produced on the surface of an ophthalmic lens which is an industrial product. Therefore, a meridian curve on which an astigmatism not larger than 0.25 diopters may appear due to inevitable errors such as manufacturing and instrumentation errors corresponds to the "umbilical meridian curve" so called in the present invention, and an ophthalmic lens having such an "umbilical meridian curve" is also included in the scope of claims of the present invention.

Claims

1. An ophthalmic lens having a progressively variable focal power, taken into account of the convergence of the eyes, wherein amount of a horizontal displacement of an umbilical meridian curve has a relationship proportional to an additional refracting power at respective positions of said horizontal displacement.

2. An ophthalmic lens as claimed in Claim 1, wherein one of two refractive surfaces of said lens includes an imaginary first meridian curve defined as an umbilical meridian curve" extending substantially in the vertical direction along said refractive surface when said refractive surface is viewed in a direction substantially orthogonal with respect thereto in the condition in which said lens stands in the same vertical direction as when it is mounted on a wearer, a set of major radii of curvature at any one of points on said umbilical meridian curve being substantially equal to each other so that the astigmatism along said umbilical meridian curve in said refractive surface is almost equal to zero, the distribution of the refractive power along said umbilical meridian curve in said refractive surface including a zone in which the refractive power increases gradually from an upper portion toward a lower portion of said curve according to a predetermined rule and in which the refractive power varies in a non-uniform manner, said umbilical meridian curve dividing said refractive surface into two lateral areas closer to the nasal side and temporal side respectively when said lens is mounted on the wearer, said refractive surface being such that, when a second meridian curve extending in the vertical direction along said refractive surface to overlap, intersect or osculate said umbilical meridian curve in an upper region of said refractive surface is imagined, said umbilical meridian curve is displaced toward the nasal side relative to said second meridian curve in a lower region of said refractive surface, while it is less gradually displaced toward the nasal side relative to said second meridian curve in an intermediate region of said refractive surface, and wherein said umbilical meridian curve is such that, in a portion of said umbilical meridian curve on which the additional refractive power varies up to at least 80% or more of a predetermined addition provided for said lens, the relation H=.Ax D+ B is substantially satisfied, where D is the additional refractive power at a point on said umbilical meridian curve, H is the displacement of said point from said second meridian curve, A is a proportional constant, and B is a constant.

3. An ophthalmic lens as claimed in Claim 1, wherein the astigmatism along said umbilical meridian curve exceeds zero but is not larger than 0.25 diopters.